Added Vagrant VirtualBox provider support for FreeBSD guest build.

Implements https://github.com/tfiala/lldb/issues/78.

Here are the errors I get with VirtualBox on FreeBSD:

Ran 313 tests.
Failing Tests (4)
FAIL: LLDB (suite) :: TestMultithreaded.py (FreeBSD packer-freebsd-10.0-amd64 10.0-RELEASE FreeBSD 10.0-RELEASE #0 r260789: Thu Jan 16 22:34:59 UTC 2014     root@snap.freebsd.org:/usr/obj/usr/src/sys/GENERIC amd64 amd64)
FAIL: LLDB (suite) :: TestSingleQuoteInFilename.py (FreeBSD packer-freebsd-10.0-amd64 10.0-RELEASE FreeBSD 10.0-RELEASE #0 r260789: Thu Jan 16 22:34:59 UTC 2014     root@snap.freebsd.org:/usr/obj/usr/src/sys/GENERIC amd64 amd64)
FAIL: LLDB (suite) :: TestStopHookMultipleThreads.py (FreeBSD packer-freebsd-10.0-amd64 10.0-RELEASE FreeBSD 10.0-RELEASE #0 r260789: Thu Jan 16 22:34:59 UTC 2014     root@snap.freebsd.org:/usr/obj/usr/src/sys/GENERIC amd64 amd64)
FAIL: LLDB (suite) :: TestConcurrentEvents.py (FreeBSD packer-freebsd-10.0-amd64 10.0-RELEASE FreeBSD 10.0-RELEASE #0 r260789: Thu Jan 16 22:34:59 UTC 2014     root@snap.freebsd.org:/usr/obj/usr/src/sys/GENERIC amd64 amd64)
ninja: build stopped: subcommand failed.

real    22m3.679s
user    9m37.833s
sys     20m17.865s

The build time looked like this:

real    88m16.704s
user    204m40.782s
sys     64m13.315s

These numbers are significantly larger than the VMware Fusion 7.0 times.  Test time was under 5 minutes with
the vmware_fusion provider (virtualbox provider looks to be more than 4x slower as currently configured).
I don't have the build time handy but it was way less than 88 minutes.